* Add fatal/futureDeprecations to Dart API
This adds a new `Deprecation` class that specifies an ID for each
deprecated feature along with what Dart Sass version deprecated it.
The compile functions allow you to pass a set of `fatalDeprecations`
that will cause an error instead of a warning. You can also pass a set
of `futureDeprecations`, which let you opt-in to deprecations (like
`@import`) early.
* Add future deprecation for `@import`
* Add flags
* Merge colorUnits and randomWithUnits
* Update changelogs and pubspecs
* Add tests
* Use isFuture instead of deprecatedIn == null
* Split warnForDeprecation from warn
* Add missing word to error message
